Bioactive macromolecules (e.g., enzymes, antigens, antibodies, and hormones) are typically produced in low concentration by normal or genetically-altered microbial fermentation broths. Undesired impurities in the broth may include large biomolecules, cell organelles, membrane lipids, amino acids, saccharolytic substrates, and a host of electrolytes. Among the industrial and labscale purification schemes which are presently used to purify bioactive macromolecules are crystallization, ion exchange chromatography, hydrophobic chromatography, electrophoresis, and affinity chromatography. See generally Bonnerjea, J. et al., Bio/Technology 4, 955 (1986). The latter technique, affinity chromatography, is by far the most selective technique but is very difficult to scale up, and the activation of the gel beads with affinity ligands is an expensive procedure that is difficult to control.
Recently, Mattiason and Ling employed particle-bound affinity ligands to specifically purify proteins from solution. See Bioprocess Technology, 99 (McGregor, W.C., ed. 1986) (Marcel Dekker, Inc., N.Y.); see also European patent application publication no. 0127737. Ligands were immobilized on starch granules or particulate heat-killed yeast cells, and subsequently mixed with the solution. After allowing affinity binding to occur, the impurities were removed by ultrafiltration through a membrane which retained the solid particles. The authors suggest that the technique can be scaled up for large-scale purification, but note that the maximum amount of macromolecules that can be bound is limited by the available surface area of the particles. In addition, the authors note that the fouling and clogging of membranes is a potential problem with this technology. Finally, the authors do not suggest any general scheme by which a variety of different ligands may be bound to the small particles they employ. Accordingly, an object of the present invention is to provide an affinity purification procedure employing small particles with large surface areas, which particles do not cause undue fouling of filters or semipermeable membranes, and which particles are convenient carriers for a variety of different ligands.